1. Field of the Invention
The present invention relates to technology for non-volatile storage.
2. Description of the Related Art
One example of non-volatile memory uses variable resistance memory elements that may be set to either low a low resistance state or a high resistance state, and can remain in that state until subsequently re-set to the initial condition. The variable resistance memory elements are individually connected between two conductors (typically bit and word lines). The state of such a memory element is typically changed by proper voltages being placed on the intersecting conductors.
Some variable resistance memory elements may be in the high resistance state when first fabricated. The term “FORMING” is sometimes used to describe putting the variable resistance memory elements into a lower resistance state for the first time. After FORMING, the variable resistance memory elements may RESET to the high resistance state and SET again to a low resistance state.
One theory that is used to explain the FORMING mechanism, as well as the switching mechanism to RESET and SET the variable resistance memory elements, is that one or more conductive filaments are formed by the application of a voltage to the variable resistance memory elements. One example of a variable resistance memory element includes a metal oxide as the variable (reversible) resistance material. In response to a suitable voltage, a conductive filament may be formed in the metal oxide such that there is one or more conductive paths from the top electrode to the bottom electrode of the variable resistance memory element. The conductive filament lowers the resistance of the variable resistance memory element. Application of another voltage may rupture the conductive filaments, thereby increasing the resistance of the variable resistance memory element. Application of still another voltage may repair the rupture in the conductive filament, thereby decreasing the resistance of the variable resistance memory element once again. The initial formation of the conductive filament is referred to as “FORMING,” the rupture of the filament is referred to as RESETTING and the repair of the rupture of the filament is referred to as SETTING. The variable resistance memory element may then be repeatedly switched between states by repeatedly RESETTING and SETTING the variable resistance memory element. The RESETTING process puts the variable resistance memory element in the high resistance state and the SETTING process puts the variable resistance memory element in the low resistance state. Data values are assigned to the high resistance state and the low resistance state.
The FORMING process may impact the ability of the variable resistance memory element to exhibit proper switching behavior. For example, the variable resistance memory element should switch between the high resistance state and the low resistance state in response to appropriate voltages, which may be referred to as “switching within the intended window.”
Current FORMING operations often use a high voltage, which can result in high transient currents. These transient currents make it difficult to manage the FORMING process and sometimes may result in damaging one or more portions of the memory device. Surge currents are very difficult to control when a resistive memory layer is being formed, especially at higher operational voltage. The resistance changes quickly, often in less than 1 ns. Array lines driving the variable resistance memory elements have capacitance and a high voltage. This capacitive energy C*V discharges through the suddenly low resistance path of the variable resistance memory element producing currents that can reach 100 micro amps (or even the mA level) and cause the low resistance state to be even lower than desired, or stresses the variable resistance memory element so that it cannot not cycle as many times as desired.